1. Field of the Invention
The present invention relates to spectrally sensitized silver halide photographic emulsions and particularly to silver halide photographic emulsions which are supersensitized by a novel combination of two sensitizing dyes.
2. Description of the Prior Art
It is well-known that the light-sensitive wavelength range of silver halide photographic emulsions can be expanded by addition of sensitizing dyes to achieve spectral sensitization. Although only one sensitizing dye is sometimes used in order to sensitize the silver halide photographic emulsion to the desired spectral sensitization wave-length, two or more sensitizing dyes are often used in combination. When two or more sensitizing dyes are used in combination, the degree of spectral sensitization often is intermediate the sensitization achieved using each sensitizing dye individually and sometimes is less than the sensitization using each sensitizing dye individually. However, when a specific combination of different sensitizing dyes is used, the degree of spectral sensitization sometimes marked by increases as compared with the case of using each sensitizing dye individually. Generally, this phenomenon is called supersensitization by the sensitizing dyes.
When such a combination is used, even though a spectral sensitization wavelength often is intermediate between or a mere combination value of the spectral sensitization wavelengths obtained using each sensitizing dye individually, a shift of the spectral sensitization to a wavelength which is not expected from the spectral sensitization achieved with each dye sometimes occurs.
Accordingly, an important subject in spectral sensitization techniques for silver halide photographic emulsions has been to discover combinations of sensitizing dyes which provide a higher spectral sensitivity than the sensitivity achieved using each sensitizing dye individually while having a sensitization wavelength range appropriate to the purpose of use of the photographic sensitive material.
In combining sensitizing dyes to achieve supersensitization, a strict selection of the dyes is required because a slight difference in the chemical structure of the dyes has a marked influence upon the supersensitization. Namely, prediction that super-sensitization will be achieved with a combination of sensitizing dyes is difficult by consideration of the mere chemical structure of the dyes.
The sensitizing dyes used in the supersensitization of silver halide photographic emulsions must not adversely interact with the other photographic additives and must have stable photographic properties during storage of the photosensitive materials.
Further, after processing the photosensitive materials, a residual color must not remain in the photosensitive materials containing the sensitizing dyes, particularly, a residual color must not be generated on processing the photosensitive materials for a short period of time (generally, about 3 to about 90 secons) such as in a rapid processing. A residual color is particularly not suitable for a photographic paper because a bright white color cannot be obtained in black-white photography and reproduction of true colors is impossible in color photography.
It is known (as disclosed in, for example, U.S. Pat. Nos. 3,752,670 and 3,480,434, German Patent Application (OLS) 2,303,204 and Japanese Pat. No. 30023/1971) to use sensitizing dyes in order to increase the sensitivity in the blue range. However, none of the sensitizing dyes used for sensitization in the blue range have sufficient photographic properties. Namely, simple merocyanine dyes as described in U.S. Pat. No. 3,480,434 and Japanese Pat. No. 30023/1971 have the defect that the spectral sensitivity distribution of the silver halide emulsion is broad and extends to a long wavelength side, and consequently a green range is sensitized. This disadvantage causes a deterioration of the color reproduction property in multilayer multicolor photosensitive materials. Although it is possible to somewhat decrease the sensitivity in the green range by providing a yellow filter on the blue-sensitive layer so as to overcome the above-described disadvantage, the sensitivity in the blue range decreases at the same time.
A certain type of monomethine cyanine dyes described in U.S. Pat. No. 3,752,670 and German Patent Application (OLS) 2,303,204 has the disadvantage of causing a high level of residual color, although this type of dye has a sharp J-band sensitization. Therefore, improved photographic emulsions wherein the longer wavelength side of the spectral sensitization range in the blue range is sharply cut, and the photographic element produced therefrom causes less dye stain after processing, must be provided.